An available bandwidth (also called “usable bandwidth”) of a communication line is an unused bandwidth obtained by subtracting other traffic (hereafter referred to as “cross traffic”) flowing in a network from a physical bandwidth of a bottleneck link in the communication line. For example, in the case where the physical bandwidth of the bottleneck link in the communication line is 100 Mbps and the cross traffic is 30 Mbps, the available bandwidth is 100−30=70 Mbps.
Estimating the current available bandwidth is important in video chat, video calling, video conferencing, and the like where communication is performed by two-way transmission of video between terminals. By limiting a transmission rate of video to not greater than the estimate of the available bandwidth, it is possible to keep the sum of the transmission rate of video and the cross traffic from exceeding the physical bandwidth of the bottleneck link in the communication line. This prevents packet loss, thus ensuring video quality.
Non Patent Literature (NPL) 1 proposes a method in which a transmitter terminal transmits a plurality of probe packets of a fixed size to a receiver terminal as one set (hereafter referred to as “packet train”), and the receiver terminal detects a change in reception interval of probe packets to estimate an available bandwidth. This method has a feature that, when the transmitter terminal transmits the packet train, the probe packets are transmitted at exponentially decreasing transmission intervals. Accordingly, in this method, a transmission rate of probe packets increases exponentially in the packet train.
In the case where the transmission rate of probe packets exceeds the available bandwidth of the network when the packet train travels through the network, probe packets are temporarily queued in a device such as a router or a switch in the network. This causes an increase in reception interval of probe packets in the receiver terminal relative to the transmission interval in the transmitter terminal. The available bandwidth is computed based on this property. In detail, the receiver terminal detects a point at which the reception interval of probe packets starts to increase relative to the transmission interval in the transmitter terminal, and divides the packet size of the probe packet by the transmission interval at the point, to compute the available bandwidth.
Patent Literature (PTL) 1 proposes a method in which a transmitter terminal transmits a plurality of probe packets of gradually increasing packet sizes to a receiver terminal as a packet train, and the receiver terminal detects a change in reception interval of probe packets to estimate an available bandwidth. This method has a feature that, when the transmitter terminal transmits the packet train, the probe packets are transmitted at regular transmission intervals. In this method, a transmission rate of probe packets increases linearly in the packet train. The receiver terminal computes the available bandwidth in the same way as the computation method proposed in NPL 1.
PTL 2 proposes a method in which a transmitter terminal repeatedly performs an operation of transmitting a packet train made up of a sequence of probe packets of a fixed size at regular transmission intervals to a receiver terminal a plurality of times, to estimate an available bandwidth. In this method, the available bandwidth is estimated through binary search by repeatedly performing the following operation: the transmitter terminal transmits the packet train at exponentially decreasing transmission intervals to the receiver terminal in the case where the reception interval tends to increase in the receiver terminal, and transmits the packet train at exponentially increasing transmission intervals to the receiver terminal in the case where the reception interval tends to decrease in the receiver terminal.